PHRAS-4042: fix webvtt field to upload to expose #927

Job	Run time
build phraseanet-fpm / build	4m 23s
build phraseanet-elasticsearch / build	20s
build phraseanet-db / build	40s
build phraseanet-nginx / build	2m 49s
build phraseanet-setup / build	3m 37s
build phraseanet-worker / build	3m 44s
unit test	5m 33s
	21m 6s

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